Recently, electronic apparatuses are becoming smaller in size and higher in density, and there is a strong demand for multi-layer circuit boards in the field of household use as well as industrial use.
In such a circuit board, it is absolutely necessary to make connections between circuit patterns of multiple layers by a via-hole connecting method, and also, it is required to assure high reliability.
In Japanese Laid-open Patent H6-268345, a high-density circuit board of novel configuration that makes via-hole connection by using conductive paste and its manufacturing method are disclosed. The method of manufacturing the circuit board is described in the following.
The method of manufacturing a conventional double-sided circuit board and multi-layer circuit board, 4-layer circuit board, will be described in the following with reference to FIGS. 4(a)-4(g) and FIG. 5.
First, the method of manufacturing a double-sided circuit board that is a base for a conventional multi-layer circuit board is described.
FIGS. 4(a) to 4(g) are sectional views showing the steps of manufacturing a conventional double-sided circuit board.
Prepreg sheet 101 is a substrate made from a composite material, non-woven cloth of aromatic polyamide fiber impregnated with thermosetting epoxy resin. For example, thickness t100 is 150 μm, and compressibility is about 35%. Also, selected as prepreg sheet 101 is a porous material having hole portions for attaining compressibility.
One side of releasing film 102a, 102b made of polyethylene terephthalate is coated with Si-based releasing agent. Through-hole 103 is filled with conductive paste 104 electrically connected to metallic foil 105a, 105b such as Cu which is affixed to both surfaces of prepreg sheet 101.
A conventional circuit board and its manufacturing method will be described in the following in accordance with the steps.
First, in the step shown in FIG. 4 (a), releasing films 102a, 102b are bonded to either surface of prepreg sheet 101. In the step shown in FIG. 4 (b), through-hole 103 is formed in a predetermined portion of prepreg sheet 101 by using a laser beam machining process or the like. In the step shown in FIG. 4 (c), conductive paste 104 is filled into through-hole 103 by using a printing method or the like. In the step shown in FIG. 4 (d), releasing films 102a, 102b are removed from the surfaces of prepreg sheet 101. And, in the step shown in FIG. 4 (e), metallic foils 105a, 105b are placed on either surface of prepreg sheet 101.
Next, in the step shown in FIG. 4 (f), a pressure is applied under heat by means of a hot press. As a result, prepreg sheet 101 is compressed to t200 (t200=about 100 μm) in thickness. Simultaneously, prepreg sheet 101 is bonded to metallic foil 105a, 105b, and metallic foils 5 on both surfaces are electrically connected with each other by conductive paste 104 filled in through-hole 103 formed in predetermined position.
Finally, in the step shown in FIG. 4 (g), metallic foils 105a, 105b on both surfaces are selectively etched to form circuit patterns 106a, 106b, thereby obtaining a double-sided circuit board.
In Japanese Laid-open Patent H6-268345, nothing is mentioned about the temperature profile of hot press. Generally, however, taking into account the number of stacks or sheets and quality, as shown in FIG. 5, widely employed are a two-stage pressurizing method in which the pressure is low until reaching a midway point of pressure rise and becomes high thereafter, and a two-stage heating method in which the temperature is kept at around 130° C. where the melting viscosity of resin component of the prepreg sheet is close to the lowest point, and the stacked multiple boards are molded with the temperature of each board nearly kept constant before increasing the temperature up to the hardening temperature.
Namely, great importance is attached to the starting temperature for hardening (molding) of thermosetting resin component in the prepreg sheet, that is, setting and keeping the temperature at a point close to the lowest viscosity due to melting of the thermosetting resin.
However, in the above conventional circuit board and its manufacturing method, if the through-hole is reduced in size and the through-hole forming pitch is narrowed in order to meet the requirement for finer circuit boards, there will arise the following problem.
That is, a prepreg sheet as porous material has hole portions for attaining compressibility. When the percentage of hole portions is high, a part of conductive paste is liable to get into the hole portions, causing the resistance of conducting hole (via-hole) to increase or the insulation between adjacent holes to worsen.
Accordingly, it is preferable to use a material of low hole percentage, but material of low hole percentage is less in compressibility. In that case, there arises a new problem. The new problem is explained with reference to FIGS. 6(a)-6(b).
As shown in FIG. 6 (a), when prepreg sheet 101 of 35% compressibility is used, conductive paste 104 is able to attain sufficient compression before resin component in prepreg sheet 101 flows in both directions, and therefore, conductive paste 104 will not flow out of the through-hole and stable connecting resistance can be obtained.
However, when a prepreg sheet of low hole percentage and low compressibility is used, that is, as shown in FIG. 6 (b), when prepreg sheet 101 whose compressibility is less than 10% is used, the compressibility of conductive paste 104 in the step of heating under pressure will become decreased and the pressure welding force of conductive particles in the conductive paste is lowered.
Thus, when the resin component in prepreg sheet 101 heated under pressure is melted and flows in both directions, paste flow 115 will take place as shown in FIG. 5. That is, conductive paste 104 flows out of through-hole 104, increasing the connecting resistance of conducting hole and worsening the quality as a circuit board.